Computer systems have evolved from the individual stand-alone units whose reliability was no better than that of the least reliable component of the entire system. There developed from these early computers operating systems that joined several computers together into a network where reliability was increased by the sheer multiplication of numbers, that is, by providing back-up computing capability to take over in case of a failure of a primary system. In control theory this is known as redundancy which, while effective to improve system reliability, is ineffective from a cost standpoint. Today there are sophisticated computer systems that incorporate the advantageous cost effective features of earlier computers but minimize the shortcomings of such computers. One such modern computer system is available from Tandem Computers Incorporated of Cupertino, Calif. This computer system of Tandem Computers Incorporated is described in a manual, Part No. 82000, dated October, 1980. The computer system of Tandem Computers Incorporated provides fault tolerance, on-line repair, and a modular design, all to enhance system reliability. The computer system of Tandem Computers Incorporated utilizes interprocessor communication over multiple processor buses thereby eliminating the need for shared memory between the various processor channels.
Fault tolerance is the availability of a system to continue operation even if a particular component or even an entire system channel fails.